In recent years, for example, liquid crystal display devices have been used widely in liquid crystal TVs, monitors, and mobile telephones, as flat panel displays having features such as thinness and light weight, compared with conventional Braun tubes. Such a liquid crystal display device includes a lighting device (backlight) emitting light and a liquid crystal panel displaying a desired image by playing a role of a shutter with respect to light from a light source provided in the lighting device.
Further, the above-mentioned lighting devices are classified roughly into a direct-type and an edge-light type depending upon the arrangement of light sources with respect to the liquid crystal panel. A liquid crystal display device having a liquid crystal panel of 20 inches or more generally uses the direct type lighting device that can achieve the increase in brightness and enlargement more easily than the edge-light type lighting device. More specifically, in the direct type lighting device, a plurality of linear light sources are placed on a rear side (non-display surface) of the liquid crystal panel, and the linear light sources can be placed right on a reverse side of the liquid crystal panel, which enables a number of linear light sources to be used. Thus, the direct type lighting device can obtain high brightness easily, and is suitable for the increase in brightness and enlargement. Furthermore, the direct type lighting device has a hollow structure, and hence, is light-weight even when enlarged. This also allows the direct type lighting device to be suitable for the increase in brightness and enlargement.
Further, in the conventional direct type lighting device as described above, for example, as described in JP 2002-231034 A, it is proposed that a plurality of cold-cathode fluorescent tubes are provided as light sources, and inverter circuits are connected to the respective cold-cathode fluorescent tubes to drive the respective cold-cathode fluorescent tubes with high-frequency lighting by the inverter circuits.
Further, for example, as described in JP 2000-292767 A, it is proposed that the conventional lighting device adjusts the amount of light incident upon a liquid crystal panel from a light-emitting plane by lighting cold-cathode fluorescent tubes using pulse width modulation (PWM), thereby controlling the lightness (brightness) on a display surface of the liquid crystal display device. More specifically, in the conventional lighting device, it is shown that a liquid crystal display device excellent in display performance (lightness) is configured using PWM dimming whose dimming range on a light-emitting plane, i.e., an adjustable brightness range is larger than that of the conventional current dimming.